Sensors or detectors for detecting movement of ferrous or magnetic objects are known. One application for such devices is in detecting the approach and retreat of features of a rotating ferrous object, such as gear teeth or poles of a ring magnet. The magnetic field associated with the ferrous object is often sensed by one or more magnetic field-to-voltage transducers (also referred to herein as magnetic field sensing elements), such as Hall elements or magnetoresistive devices, which provide a signal proportional to a sensed magnetic field (i.e., a magnetic field signal). The sensor processes the magnetic field signal to generate an output signal that changes state each time the magnetic field signal crosses a threshold signal. Therefore, when the sensor is used to detect the approach and retreat of each tooth of a rotating ferrous gear for example, the output signal is a pulse waveform representative of rotation of the ferrous gear and can be used to provide rotational speed information.
Sensors may use a peak-to-peak percentage detector in which the magnetic field signal is compared to a threshold signal that is a percentage of the peak-to-peak magnetic field signal. One such peak-to-peak percentage detector is described in U.S. Pat. No. 5,917,320 entitled “Detection of Passing Magnetic Articles while Periodically Adapting Detection Threshold,” which is assigned to the assignee of the present invention.
Sensors may also use a slope-activated or a peak-referenced detector, one type of which is described in U.S. Pat. No. 6,091,239 entitled “Detection of Passing Magnetic Articles with a Peak-Referenced Threshold Detector,” which is assigned to the assignee of the present invention. Another such peak-referenced detector is described in U.S. Pat. No. 6,693,419 entitled “Proximity Detector,” which is assigned to the assignee of the present invention. In the peak-referenced detector, the threshold signal differs from the positive and negative peaks (i.e., the peaks and valleys) of the magnetic field signal by a predetermined amount. Thus, in this type of proximity detector, the output signal changes state when the magnetic field signal comes away from a peak or valley by the predetermined amount.
In order to accurately detect the ferrous object, the detectors must be capable of closely tracking the magnetic field signal. Typically, one or more digital-to-analog converters (DACs) are used to generate a DAC signal, which tracks the magnetic field signal. For example, in the above-referenced U.S. Pat. Nos. 5,917,320 and 6,091,239, two DACs are used, one to track the positive peaks of the magnetic field signal (PDAC) and the other to track the negative peaks of the magnetic field signal (NDAC). And in the above-referenced U.S. Pat. No. 6,693,419, a single DAC tracks both the positive and negative peaks of the magnetic field signal.
In some applications, it is desirable to detect not only rotation of a target and/or the speed of rotation, but also the direction of rotation. One way of providing a direction signal requires the use of two magnetic field transducers spaced apart from each other. In some such arrangements, the output signals of the two transducers are processed to provide respective pulse signals which are compared to determine the relative phase of the signals which in turn is indicative of the direction of rotation. An example of such an arrangement is a sensor that is sold under a part no. 3422 by Allegro MicroSystems, Inc. of Worcester, Mass., the Assignee of the subject application.
Another way of providing a direction signal using only a single magnetic field transducer is described in a Japanese Unexamined Patent Application Publication H2-116753. In this arrangement, the target has an asymmetric shape and a signal provided by a pulse generator is processed by a waveform shaping circuit to detect the direction of rotation based on the polarity of the maximum output value of the voltage generator.
The magnetic field associated with a ferrous object and the resulting magnetic field signal are proportional to the distance between the ferrous object, for example the rotating ferrous gear, and the magnetic field transducer(s). This distance is referred to as the “air gap.” As the air gap increases, the magnetic field transducer(s) tend to experience a smaller magnetic field from the ferrous object, and therefore a smaller amplitude magnetic field signal results.